Virtual image display device

ABSTRACT

A head-coupled display device for use in presenting electronically generated visual images to a viewer. The device includes a headpiece worn by a viewer, and an image display screen mounted in the headpiece for presenting electronically generated images. A fresnel lens is positioned adjacent the screen for focusing images from the screen at a selected position for viewing as a virtual image. The lens has a set of concentric converging rings whose surface curvatures act to minimize rectilinear distortion of the virtual images perceived by the viewer over the entire field of view of the image.

This application is a Continuation of application Ser. No. 08/141,204,filed on Oct. 6, 1993, now abandoned, which was a Continuation ofapplication Ser. No. 07/725,032, filed on Jul. 3, 1991, now abandoned.

FIELD OF THE INVENTION

The present invention relates to a virtual image display device.

BACKGROUND OF THE INVENTION

Virtual reality systems are designed to present a user withcomputer-generated visual, and optionally, auditory cues which allow theuser to experience a computer-simulated reality. At the same time, theuser may be equipped with a motion-responsive glove or body suit whichpermits user interaction with the virtual reality information beingpresented. For example, the user may be able to touch and "move" anobject in the visual field, or experience a changing auditory cue as theimage of an object in the visual field is moved with respect to theuser's ear.

One important application of virtual reality systems is in flightsimulators, where visual flight information is provided to atrainee-user. Another is for actual in-flight guidance, where visualcues processed from infrared or radar signals are provided to one of thepilot's eyes. Other applications are emerging, for example inarchitecture, medicine, engineering, and entertainment, as real-timecomputer-graphics capabilities are developed.

One desired type of virtual reality system employs a head-mounteddisplay (HMD) designed to be worn by a user, and capable of generatingvisual images which are perceived as three-dimensional virtual images.Head-mounted displays for use in flight simulation systems have beenproposed. For example, U.S. Pat. No. 4,048,653 describes a head-coupledvisual display system which includes a head-mounted unit worn by theobserver, and a image projection system for projecting left-eye andright-eye images onto left- and right-eye screens in the head-mountedunit, via an optical train. More recently, the present inventors havedescribed a HMD in which visual images are generated by liquid-crystaldisplay (LCD) screens in a head-mounted unit, and viewed by the observeras three-dimensional virtual images.

In general, it is desirable for an observer to experience a wide-anglevirtual image, such that even the peripheral vision of the observer isengaged. One aspect in the design of a wide-angle visual system in anHMD is the requirement for a large-diameter lens having a relativelysmall focal length. Such lenses can be relatively bulky and expensive.Further, spherical aberration effects associated with spherical glasslenses can produce pronounced rectilinear distortion at the outerregions of the wide-angle image. Heretofore, although a variety ofhead-coupled image systems have been proposed, none of these has provencapable of achieving distortion-free wide-angle image presentation.

SUMMARY OF THE INVENTION

The invention includes, in one embodiment, a head-mounted display devicefor use in presenting electronically generated visual images to aviewer. The device includes a headpiece adapted to be worn by viewer,and an image display screen mounted in the headpiece for presentingelectronically generated images. A fresnel lens is positioned adjacentthe screen for focusing images from the screen at a selected positionfor viewing as a virtual image which is a selected distance from theviewer's eye. The lens has a set of concentric converging rings whosesurface curvatures act to minimize rectilinear distortion of the virtualimages perceived by the viewer over the entire field of view of theimage. Preferably the device includes such a screen and lens assemblyfor both left and right eyes, for viewing stereoscopic images.

For viewing images over a wide angle, e.g., 60° or more, the lens ispreferably a compound lens includes a pair of fresnel lenses positionedfront-to-back. The lens diameter is preferably 60 mm or greater indiameter, and the focal length is preferably 30-50 mm or less.

In one preferred embodiment, the lens is about mid-distance between thedisplay screen and the viewer's eye, in operative condition, and thevirtual image produced by the lens is at least about 10 feet in front ofthe viewer's eye.

Also in a preferred embodiment, the image display screen is a liquidcrystal display screen having an array of pixels which make up clustersof RBG pixel groups. The device further includes a partially translucentplate positioned adjacent said screen, effective to blur the distinctionbetween the individual pixel elements in such as RBG pixel cluster.

In a more general aspect, the invention includes a head-coupled imagedisplay device for use in viewing an electronically generated images asa virtual image. The assembly includes an image display screen forpresenting electronically generated images, and a fresnel lens forfocusing images from the screen at a selected position for viewing bythe viewer as a virtual image which is a selected distance from theviewer's eye. The lens has a set of concentric converging rings whosering curvatures are designed to produce a desired light-refractioneffect, such as reducing outer-field rectilinear distortion over awide-angle view field. The screen and lens are carried on a head-coupledstructure, such as a glasses frame, or a hand-held frame, forpositioning the viewer's eye at the near conjugate of the lens.

These and other objects and features of the invention will become morefully apparent when the following detailed description of the inventionis read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a head-mounted display device containingthe display optics of the present invention;

FIG. 2 shows the display optics for one eye in the head-mounted deviceof FIG. 1;

FIG. 3 shows a portion of the face of a liquid crystal display array inthe display optics;

FIG. 4 shows the face of a fresnel lens in the display optics;

FIG. 5 is a ray diagram illustrating how the fresnel lens assemblyillustrated in FIG. 5 acts to reduce rectilinear image distortion atouter-field regions of the image screen; and

FIG. 6 illustrates how a colored image is produced by the displayoptics.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a head-mounted display (HMD) or display device 10constructed according to one embodiment of the invention. The devicegenerally includes a headpiece 12 designed to worn by a viewer, and anoptical assembly 14 for producing wide-angle, electronically generatedvirtual images to each eye of the viewer, as described below. Also shownin the device, although not forming part of the present invention, is apair of earphones, such as earphone 16, which are used in sendingauditory cues, preferably linked to the observed visual images, to theuser.

The headpiece includes a frame 17 designed to fit on the viewer's headand a mask 18 which fits over the viewer's eye region. Although thestructure used in coupling the optical assembly to a viewer's eye in theFIG. 1 embodiment is a headgear which engages front, back, and sides ofthe user's head, other types of head-mounted structures, such as aglasses-type frame are contemplated.

In a more general embodiment, the optical assembly is carried on asupport structure, such as a desk-mounted support, or a hand-heldsupport which functions to hold the optical assembly at a relativelyfixed position with respect to the viewer's eyes, for viewing a virtualimage produced by the assembly. For example, a desk-mounted supportmight include a base mounted on the desk, and a movable arm which ispositionable to place the optical assembly, which is carried on the arm,in position for viewing.

With reference to FIGS. 1-3, the optical assembly in the device includesan image display screen 20 mounted in the headpiece for presentingelectronically generated images to the viewer. In the embodimentillustrated, the image display screen is produced by a pair of liquidcrystal display (LCD) units 22, 24. According to standard construction,each LCD unit includes a source of unpolarized light, a first polarizer,a polarization rotating array (which creates individual pixels), and asecond polarizer which serves as an analyzer. (These components arerepresented by a single plate in the FIGS. 1 and 2).

The cutaway of device 22 in FIG. 1, and FIG. 3, shows an array 26 ofpixels 28, which form the image display screen for the user's right eyein the device. LCD units having a 62×48 mm square pixel array, and about102,000 pixels per array are commercially available, such as from Epson(Japan).

Electronically generated images are supplied to the two image screensfrom a suitable image generator (not shown) connected to the LCD unitsthrough a suitable link, such as indicated at 29. The image generatormay be, for example, a pair of graphics workstations, such as SiliconGraphics workstations, designed for generating position-dependent imageswhich (a) change to reflect the relative position of the headpiece wornby the viewer, and (b) are reconstructed by the viewer as athree-dimensional image. Such image generators can be assembled fromcommercially available work station and graphics software components.

Alternatively, the image screen for presenting electronically createdimages could be provided by one or a pair of CRT's which may be mounted,for example, at the sides of the headpiece and which project theirimages via a lens or optical fiber bundle to a suitable viewing screenpositioned at the front region of mask 18, or other types of displayscreens for presenting electronically generated images. Other devicesfor producing electronically generated images, such as video cameras orthe like, are also contemplated.

The screen at which images are presented to the viewer is preferablylocated in the mask at a position which is about 5-10 cm from the planeof the viewer's pupils. The distance between the screen and the viewer'seyes will be dictated by the optical properties of the lens(es) employedin the optical assembly, and the selected positions of the conjugates inthe optical assembly, i.e., the selected focal positions on oppositesides of the lens, as will be described below.

Although the screen in device 10 is positioned directly in front of theviewer, it will be appreciated that any screen configuration which iseffective to generate an image in front of the viewer is suitable. Forexample, the screen may be placed at the top of the mask, with thescreen images projected downwardly onto a half-silvered mirrorpositioned to direct a portion of the image directly toward the viewer'seyes. Such a configuration would allow the viewer to view theelectronically generated image as well as to see "through" the image. Inthis configuration, the fresnel lens desribed below is preferablypositioned between the screen and the half silvered mirror.

Positioned adjacent the viewing side of the LCD device is a slightlycloudy or rough-surfaced film 29 which acts to blur light fromindividual pixels, and reduce viewer distraction due to individual lightarray elements seen close to the eye. The ability of the film to blurindividual elements is also useful in a color LCD device, for diffusingdifferent color pixels into larger multi-color elements, as will bedescribed below with reference to FIG. 6.

A variety of semi-translucent film material, such as velum, polyethylenesheet material with a slightly roughened surface, or material with aninternal light-scattering polymer. The film is preferably 1 to 2 mm fromthe image screen (pixel array) of the LCD device.

According to an important feature of the invention, the optical assemblyincludes a fresnel lens, such as the compound fresnel lens shown at 30in FIGS. 1 and 2. In the embodiment shown, compound lens 30 includes twoidentical fresnel lens elements, or lenses, 32, 34 which are disposed ina front-to-back arrangement as seen in FIG. 2. That is, the front (upperside) of lens 34 in the figure faces the back (lower side) of lens 32.The purpose of the two-lens configuration is to decrease the focallength of the compound lens by about 1/2, at a selected lens diameter,to produce a view angle from the viewer's eye of at least about 60°. Thetwo lenses are separated by a distance of about 2-3 mm, with theiroptical centers aligned. The spacing between the two lenses is effectiveto minimize moire pattern effects from the surface patterns of the twolens.

Fresnel lens 32, which is representative, will be described withreference to FIG. 2, which shows the lens through a diameter crosssection, and FIG. 4, which shows the square lens in planar view. Thelens is formed of a series of concentric rings, such as rings 36, 38,whose surface curvatures form a parabolic lens surface, as indicated bydash-dot line 40 in FIG. 2. In this embodiment, the surface curvaturesof the lens rings act to reduce rectilinear distortion effects--eitherpincushion or barrel type distortions--seen at outer regions of opticalimages formed by spherical lenses, at an infinite conjugate. The effectof lens curvature on rectilinear distortion will be seen below withrespect to FIG. 5.

More generally, the lens curvatures are selected to achieve a desiredimage-ray refraction effect, for producing desired corrections or imageeffects in the virtual image, at selected regions of the image. In theembodiment illustrated, the lens ring curvatures are corrected forspherical aberrations at an infinite conjugate, i.e., wheresubstantially parallel rays are focused at the lens focal point, andwhere the near conjugate corresponds to the focal point of the lens. Forfinite conjugates, different ring curvatures would be required to reducerectilinear distortion, as will be appreciated from FIG. 5 below. Thus,in a more general case, to correct for rectilinear distortion, the lensring curvatures are selected to correct for spherical aberration at agiven conjugate pair, and the eye is placed at one of the conjugates,usually the near conjugate.

Alternatively, the fresnel lens ring curvatures can be selected tocorrect or compensate refraction effects produced by other lens elementin a compound lens. For example, in a compound fresnel lens containing aconventional glass optical element, the fresnel lens ring curvatures maybe selected to correct or compensate for spherical aberration effectsdue to the glass lens element. As defined herein, "fresnel lens" maymean a single fresnel lens, a compound fresnel lens, such as shown inFIG. 2, and a compound lens containing both fresnel lens elements(s) andlens element(s) with continuous surface curvatures, such as standardglass lens elements.

Fresnel lenses having a selected surface curvature (ring curvatures) canbe formed conventionally from a master lens formed by an optical lathe.Subsequent duplications are prepared readily by embossure or castingprocesses. Suitable lenses for use in the lens configuration shown inFIG. 2 are available from Fresnel Technologies (Ft. Worth, Tex.),catalog lens no 4.

Each of the fresnel lenses forming the compound lens in the device has apreferred focal length of about 60 mm or less, and a preferred diameterof about 65 mm or greater. As indicated in FIG. 2, the square lens hasroughly the same dimensions as the image screen, for receiving andfocusing pixel information over the entire array. When the identical twolens are placed in tandem, as illustrated in FIG. 2, the focal length ofthe compound lens is approximately 1/2 that of each individual lens, andpreferably less than about 50 mm, more preferably about 30 mm or less.At the exemplified lens parameters, i.e., a 65 mm lens diameter and a 30mm focal length, the field of view is about 90°.

The compound lens is placed in mask 18 with its near conjugate at ornear the expected position of the viewer's pupil. The distance betweenthe lens and the image screen is determined by the desired viewingconjugates, i.e., the desired position of the virtual image from theviewer. In the embodiment shown, the image conjugate is the infiniteconjugate (at least about 10 feet from the viewer) which is achieved byplacing the lens one focal length from the image screen, e.g., about 30mm. By moving the screen further from the lens, the image conjugate isreduced, having the effect of moving the virtual image closer to theviewer. The mask 18 illustrated in FIG. 1 is capable of holdinginterchangeable fresnel lens or lens combinations, to achieve differentselected imaging effects with the different lens surface curvatures.

FIG. 5 illustrates how the optical assembly of the invention focuses animage from the image screen onto the viewer's eye. In the assemblyshown, the compound fresnel lens 30 of FIG. 2 has been replaced by asingle fresnel lens 32 for simplicity, it being understood that thecompound lens functions identically but with one-half the focal length.Also in this assembly, the lens is positioned one focal length fromimage screen 20, as indicated, for producing a virtual image at leastabout 10 feet from the viewer.

The figure shows the position of the viewer's eye, where the pupilaperture is indicated by the opening 42. The pupil aperture is typicallyabout 3 mm in diameter, with the focal point of the lens, indicated at43, being position at the center of the aperture.

According to an important feature of the lens design in the FIG. 1embodiment, the effective lens ring surface curvatures act to minimizerectilinear distortions which can occur at outer regions of the visualfield. This feature is achieved in a fresnel lens whose ring surfacecurvatures collectively define a parabolic lens surface facing the imagescreen, and which act to focus onto the viewer's pupil, light rays whichare directed substantially normal to the lens surface, as illustrated inFIG. 5.

In the figure, two equal-length arrows 54, 56 are formed by image screen20 at center and edge regions of the screen, respectively. The lightrays making up the center arrow, and indicated at 57, are refracted atthe center portion of lens 32, where lens curvature is substantially thesame for both spherical and parabolic surfaces. The two converging rays58, 60 shown focused into the pupil in FIG. 5, represent the chief raysfrom left and right pixels in the screen which form the rays making upthe arrow.

To construct the virtual image perceived by the viewer, the rays 58, 60are projected, in the present embodiment, to an infinite conjugate,i.e., at least about 10 feet in front of the user, to give a virtualimage 62 of the arrow. Similarly, the virtual image constructed from thelight rays making up arrow 56, is constructed from refracted rays 64, 66formed by chief rays striking the lens in a substantially normaldirection and refracted into the viewer's pupil, as in FIG. 5. Rays 64,66, when projected to the infinite conjugate, give the virtual image 67of the arrow having substantially the same dimensions as image 62 of thecenter arrow. That is, the perceived rectilinear size of images formedon the screen is substantially independent of the position on the screenwhere the images are formed.

Also illustrated in the figure is the rectilinear distortion in imagesize which would occur if outer-field images from screen 20 were focusedby a conventional spherical mirror, such as indicated by lens surface 68in dash-dot lines in the figure. It is assumed here that the sphericalaberration produced by a spherical lens would act to focus light raysfrom outer-field images, and directed normal to the lens surface, at apoint closer to the lens than at the true focal length. Accordingly,rays which are focused at the focal point (through the viewer's pupil)would originate from rays which strike the lens at a slightly obliqueangle, as shown in dash-dot lines at 71.

When the refracted rays, shown by dash-dot lines 72, 74, are projectedto the infinite conjugate, the image 76 of arrow 56 which is formed issubstantially longer than image 62 of the center arrow. This type ofrectilinear distortion in which outer-field images show distortedenlargement, is known as pincushion distortion. A reverse type ofdistortion, known as barrel distortion, would occur if aberrations inthe lens surface caused outer-field images to be focused behind theactual focal point of the lens.

In the configuration shown, rectilinear distortions are minimized byforming a fresnel lens (or pair of lenses) whose ring curvatures (a) arefacing the image screen, and (b) define a substantially parabolic lenssurface. Other fresnel-lens surface curvatures would be required tocorrect for rectilinear distortion in other lens configurations, such aswhere the lens is used in combination with a glass lens which exhibitsspherical aberrations. In the latter case, the fresnel lens ring surfacecurvatures would be selected to compensate for spherical aberrationproduced the glass lens, e.g., by a lens ring curvatures which have anexaggerated parabolic curvature in the outer lens rings.

FIG. 6 illustrates an optical assembly 80 like that described above, foruse in projecting color images to the viewer. The figure shows a portionof an LCD array 82 containing groups of RBG pixels forming three-pixelcolor groups, such as a group formed of pixels 84, 85, 86 in array 82. Apartially translucent or cloudy plate 90 in the assembly, functions, asdescribed above, to blur the distinction between individual pixels inthe image screen, and particularly in the present embodiment, to blurthe distinction between adjacent different-color pixels, to produce adiffuse color "element", such as indicated at 88 on the plate, which isformed of the three contributing color pixels.

The color elements are focused, as above, by a fresnel lens 92 onto aviewer's pupil, for producing a colored virtual image with minimizedrectilinear distortion.

From the foregoing, it will be appreciated how various objects andfeatures of the invention are met. The optical assembly provides alarge-diameter, small focal length lens for wide angle viewing. At thesame time, the fresnel lens can be designed in lens ring curvatures toeliminate or minimize rectilinear distortion effects, or create otherselected image-refraction effects. The invention thus overcomes alimitation in head-coupled display in achieving wide-angle, relativelydistortion free image presentation to a viewer. Another importantadvantage, in a head-mounted device, is the relatively light weight ofthe lens and lens assembly.

Although the invention has been described with respect to particularembodiments, it will be apparent that various changes and modificationscan be made without departing from the invention.

It is claimed:
 1. A head-coupled display device for use in presentingelectronically generated wide angle visual images to a viewer,comprising:a headpiece to be worn by viewer; an image display screenmounted in the headpiece for presenting electronically generated images;and a fresnel lens for focusing said electronically images from thescreen as virtual images, said lens having a set of concentricconverging rings whose surface curvatures are designed to correct forspherical aberration at an infinite conjugate and a near conjugate, thenear conjugate corresponding to a position of the viewer's eye, theconcentric converging rings of the fresnel lens being on a side of thefresnel lens facing the image display screen, wherein the fresnel lensis interchangeable with fresnel lenses having different effective lenscurvatures.
 2. The display device of claim 1, wherein the fresnel ringcurvatures define a parabolic surface.
 3. The display device of claim 1,wherein said fresnel lens is a compound lens which includes a pair offresnel lenses positioned front-to-back.
 4. The display device of claim3, wherein the lens diameter is at least 60 mm, and the focal length isno greater than about 50 mm.
 5. The display device of claim 4, whereinthe field of view is at least about 60°.
 6. The display device of claim1, wherein the image display screen is a liquid crystal display screenhaving an array of pixels which make up clusters of RBG pixel groups,and said device further includes a partially translucent platepositioned adjacent said screen, effective to blur the distinctionbetween the individual pixel elements in such a RBG pixel cluster. 7.The display device of claim 1, which includes two such display screensand fresnel lenses, for left and right viewer eyes.
 8. A head-coupleddisplay device for use in presenting electronically generated wide anglevisual images to a viewer, comprising:a headpiece to be worn by theviewer; an image display screen mounted in the headpiece for presentingelectronically generated images; and a fresnel lens for focusing saidelectronically generated images from the screen as virtual images, saidlens having a set of concentric converging rings whose surfacecurvatures are designed to correct for spherical aberration at aninfinite conjugate and a near conjugate, the near conjugatecorresponding to a position of the viewer's eye, wherein the fresnellens is interchangeable with fresnel lenses having different effectivelens curvatures.
 9. An optical device for viewing electronicallygenerated images, comprising:an image display screen for presentingelectronically generated images; a fresnel lens for focusing saidelectronically generated images from the screen as virtual images, saidlens having a set of concentric converging rings whose surfacecurvatures are designed to correct for spherical aberration at aninfinite conjugate and a near conjugate, the near conjugatecorresponding to the position of the viewer's eye; and a head mountedsupport for supporting the screen and lens, wherein the fresnel lens isinterchangeable with fresnel lenses having different effective lenscurvatures.